1. Field of the Invention
The present invention is generally concerned with satellite telecommunication networks. To be more precise, the invention concerns a method of regulating the power of a first signal transmitted via a satellite from a sending or first station to be received in the form of a second signal by a receiving or second station.
2. Description of the Prior Art
The radio communication channel set up between two stations in a satellite telecommunication network causes variation in both (a)--the received signal level and (b)--the received noise level at the receiving station. The variation in the received signal level is due to the characteristics of the channel. It is the result of fading due to the presence of obstacles, for example, or multiple propagation paths conveying signal components that combine incoherently with the main component of the transmitted signal. The variation in the received noise level is also due to the characteristics of the channel, as with the signal. It can also be the result of variations in the power of the noise source. Thus the signal, so-called first signal, which is transmitted by the sending or first station is transformed into a signal, so-called second signal, when received by the receiving station or second station.
The quality of the link set up through the channel deteriorates if nothing is done to remedy this problem of variation in the received signal components.
A simple solution is to allow a priori for the maximal fluctuations to which the received signal may be subjected. The sending station then sends with a power margin that guarantees the quality of the link regardless of the propagation and interference environment associated with the channel. This solution leads to overrating of the transmission system, to a loss of capacity and to overrating of the power transmitted by the stations.
In the context of land mobile networks, the fluctuations of the signal and noise components in the channel are considerable, possibly as much as several tens of dB, and occur at very high speed. Control or regulation of the power at which the signal is transmitted by the sending station is therefore restricted to correcting the average attenuation of the channel.
A prior art "closed loop" method of controlling the power of a signal transmitted by a sending station is based on the following principle: the receiving station measures the signal-to-noise ratio of the signal received from the sending station. The measured signal-to-noise ratio is sent from the receiving station to the sending station in the form of a message. The latter station corrects the power level of the transmitted signal according to the measurements carried out by the receiving station. The application of this method to the satellite networks mainly used until now, namely networks using satellites in geostationary orbit at an altitude of about 36 000 kms, would seem to be of only limited benefit. In any network of this kind, the round-trip propagation time for a signal between the sending and receiving stations is substantially equal, ignoring processing times, to the reaction time of the power regulation system, which separates the time of transmission of data by the sending station from a subsequent time of correction of the power at which data is transmitted on the basis of measurement of the received signal. For a satellite at around 36 000 kms, the reaction time of the power correction system is approximately 500 ms, which imposes an absolute limit on the possible correction of variations in components of the signal to frequencies below (1/0.5)=2 Hz. In practise, given the limitations due to the system, this range can be deemed to have an upper limit of 1 Hz.
The prior art therefore provides no effective method of power control using a "closed loop" type method. A first object of the invention is therefore to provide a method of this kind that is particularly suitable for radiocommunication networks using non-geostationary satellites in low Earth orbit. Nevertheless, the method is applicable to networks using satellite(s) in circular intermediate orbits, geostationary orbits, elliptical orbits, etc. A second object of the invention is to provide a set of two stations for implementing the method of the invention. Still another object of the invention is to provide stations for carrying out the method.